ELECTRONIC CIRCUITS NAVSHIP '5 9CQ,OW.102 FREQUENCY CONVERTERS and produces a l-mc beat signol, which rides on the locol- oscillator carrier somewhat Like a ripple voltoge. The output circuit contains pulses for each positive cycle of the two input frequencies, plus the beat signol. Selection of the desired output frequency is obtained by using a tuned circuit as the load. The meatest output is developed, of course, at the beat frequency to which the lood is tuned, and the other frequencies ore effectively rejected. The manner in which the conversion efficiency varies with local-oscillator signol amplitude is shown gro- phicolly in the accompanying figure. As can be seen the best efficiency is obtoined for the strongest lacal- oscillator signol that can be handled by the diode. Since the detector output increases very slowly after the maximum inflection point of the characteristic curve is reached, the efficiency changes very little beyond this point. Thus there is a llmit beyond which not much change of efficiency occws it the local-oscillotor power is increased. Note, however, that for very smoll signals the efficiency is low. In addition to low conversion efficiency when the input signal is of the same amplitude as the local-oscillator signol, extreme distortion is produced, because the beat signal voiies from zero to a maximum of twice normal (in effect 100% modulation). In this case the circuit act; somewhat like a square-law detector. Little use is made of this circuit charocteristic. Normally, the locoluscillotor signal amplitude is fixed at about ten times the amplitude of the received siqnal. CONYERSiON EFFICIENCY 0 L 0 PWR MAX Convrrrion Efficirncy Variation It is interesting to observe that the local-oscillotor signal is always unmodulated; if bath signals were modulated, the linear transfer of modulation would be annulled because of phase concellotion between modulation components. The normal transfer of modulotion -- -~~ ~~ occurs linearly even thaugh the bwt is produced by deliberately distortinq the r-f siunol. This deviation from the normal rule of avoiding distortion is due to the large sewrotion in frequency between the modulation (usually an audio-frequency signol) ond the input and localascillator signals (which are always radio-frequency signals). The audio modulation varies at such a relatively slow rote that many r-f cycles can be lost without a noticeable change in fidelity; that is, any change in the amplitude of a single r-f cycle has little effecl on the overall operotion. ORIGINAL Since the semiconductor diode will conduct in o reverse direction, :t represents a lower-impedance lmd than the conventional electron tube diode, and it also has o lower forward resistance. Neither porameter, however, has any greot effect on the operation of the diode as a mixer. FAILURE ANALYSIS. No Output. If the local-oscillator signal or the r-f input is missing, no i-f output will be produced. Boh signals must be present to obtain frequency conversion. An open circuit in the inout transformer (TI) . . or the lacof-oscillator transformer (T2) - either primary or secondary - will also render the circuit inoperative. A defective crystal may be checked by making forward- and reverse-resistance checks of the diode. The reverse resistance shouldbe very much larger than the forward resistance. Since the proper functioning of transformer T3 is necessary to select the output signal, an open circuit in this transiormer could also result in no output. Moreover, it is possible that mistuning of the input circuits could cause absence of one of the signals and produce a no-output indication. A resistance or continuity check of the transformers should be made to insure that the circuit is comp te, and a test signal should be ap plied to the input i lterdyne with the local-oscillator signol while checks made for an output signal in T3. It will be necessary ti ;e a VTVM with on r-f probe to determine whether the i-f voltage is present. With a modulated input to the mixer, on oscilloscope with an r-f probe can be used ta abserve whether the modulation appears in the output. Low Output. A Iaw-aut~ut condition con occur if the local-oscillator signal is Gual to or less than that of the input siqnal. Such o condition could becousedbv mistking oi TI or T2. Substituting a test siqnol in' place of the locol-oscillator signal will quickly determine whether the oscillator output is low (if the proper value is known, the oscillota voltage can be measured with o VTVM and r-f probe). A similar result could be produced by the lccal-oscillator being tuned off frequenq as o result of improper padding or alinement. Other Condltlons. Defective shielding or lack of proper lead dress after repair moy permit the 1x01- oscillator signal to leak into the preeding stages of the receiver and reduce the receiver sensitivity by producing a high avc voltoge. Or it may allow the local-oscillator signal to feed back through the i-f and detector staqes and cause birdies and unwanted signals. Such troubles will disappear when the local oscillator is temporarily disabled and a well shielded test-signal source is substituted in its place.
ELECTRONIC CIRCUITS NAVSHIPS TRIODE MIXER APPLICATION. The tracsistoi triode nixei is used in trmslstorized siiperheterodyne receivers to combine the ixmminq i-f signal with the local oscillator siqnd to produce the de- -. . - - . . . - -. , - , . c
Change language